How to add keyframes for multiple objects at once
Assuming you're talking about actual objects (not a mix of objects and armature bones), the easiest thing would be to select all of the objects and, from within the 3D View, press I and choose LocRot. That will insert a key for location and rotation on all selected objects.

If your needs are more nuanced, you may want to consider building a custom keying set. In that, you can specify any number of different properties from any number of objects (and non-objects... like World properties) and key them all at the same time. If you're interested in taking that route, then Getting to Grips with Keying Sets is a good start.